Sports racquets, which term includes tennis rackets, squash racquets, badminton racquets and racquetball racquets, are all strung with strings across a head portion of a frame, which head portion surrounds and defines a string bed. The string bed is designed to intercept and return a game piece such as a shuttlecock, racquetball or tennis ball.
Up into the 1960's sports racquets were made of wood. These racquets were replaced with racquets made of metal, typically of aluminum alloy, although steel has also been used. In the 1970's thermoplastic injection molded racquets were attempted, as reinforced with fiber whiskers. Also in the 1970's sports racquets began to be made from a composite material which has as its basic constituents (a) plural laminations of fibrous material such as carbon fiber, boron, fiberglass and/or aramid compositions, and (b) a binding thermosetting resin. While each succession of materials in general improved strength to weight ratios, the engineering problems associated with them differ markedly.
Racquets made from aluminum and related nonferrous alloys are made from extruded tubes, I-beams and like shapes, with or without internal reinforcing walls. The cross-sectional shape of the frame member is dictated by the extrusion die. The extrusion process permits tight control of the positioning of internal bridges, struts and reinforcements. Straight sections of aluminum extrusion may be stamped with drill positioning dimples, and with dimples or grooves to create space for strings, bumpers and handle parts. The straight extrusion may have sections of it crimped to vary the cross-section shape. The straight extrusion is then formed into a racquet frame by bending.
While forming racquet frames from extruded aluminum alloys is relatively cheap because of lower labor costs, the material has many limitations. An extruded metal cross-section cannot be altered with processes such as welding, crimping or pressing without weakening the strength of the original extruded structure. It is therefore common to have little or no variation in cross sectional shape along the length of the frame. Aluminum extrusions have substantial weight limitations. There may be areas along the frame which require additional strength or flexibility to limit breakage or improve playability. To effect changes to these areas while not weakening the frame, typically the cross-sectional shape along the entire length of the extrusion is changed. Those regions which did not require reinforcement are nonetheless made heavier.
Conventional composite frames are formed in molds. In the most common manufacturing process, a “layup” is created by applying multiple sheets or laminations, commonly formed of fibrous material such as carbon fiber, to a single bladder. The bladder in turn contains a rigid mandrel to control the desired layup shape. The sheets are pre-impregnated with a thermosetting resin prior to their application to the layup. This layup is placed in a mold and the mold is closed. The bladder is inflated with a single air nozzle to force the walls of the layup to the interior walls of the mold and the mold is then subjected to a thermal step. An artifact of this process is that composite racquet frames are commonly of a single-tube design. While there have been multiple-tube composite structures, it has been found that any internal divisions, bridges or lumens placed in these tubes are difficult to control in their placement because of variations in bladder air pressure, and attempts to include them in the past have been found to cause significant quality control and production problems.